They could have built a Shanghai or even WDW sized resort given the right location in the UAE. It would have been a long-term deal and required quite a bit of investment, but they could have done it. Building it on Yas Island is the easy way out. They will build it and you will get at least 5 or 10 million visitors minimum from Disney, probably more if they would have legitimately tried to build it out themselves instead of making a relatively small investment. This pigeonholes them in a way that I'm honestly a little shocked by location-wise. Can't really fit an entire WDW style resort there unless they bulldoze other already existing parks!
As I noted elsewhere, this will legitimize the UAE as a theme park destination, and will skyrocket their visitor numbers. This isn't about a current visitor play, this is about 10 years from now. It's actually really smart, arguably would have been even better 10 years ago. (But I also might be a little biased because I've been studying the tourism industry in the UAE and Gulf monarchies for years and have been saying that Disney is going to go to the Middle East for probably the last 10 minimum.)